The invention relates to a scanning device for optically scanning a first record carrier having a first information plane and a first transparent layer of a first thickness and for scanning a second record carrier having a second information plane and a second transparent layer of a second thickness different from the first thickness, comprising a radiation source for generating a radiation beam and an objective lens for focusing the radiation beam through one of the transparent layers on one of the information layers.
The transparent layer in optical record carriers has, in general, the function of protecting the information layer against environmental influences and providing mechanical support for the record carrier, i.e. it acts as a substrate for the information layer. The thickness of the transparent layer is a compromise between the desired stiffness of the record carrier and the numerical aperture of the radiation beam used for scanning the information layer. If for a new type of record carrier the numerical aperture is increased in order to increase the storage density of the information layer, it is often necessary to reduce the thickness of the transparent layer in order to reduce the influence of disc tilt on the quality of the radiation beam. As a consequence, there will be different types of record carrier on the market, having different thicknesses of the transparent layer. A compatible record player should be able to scan all types of record carrier, irrespective of the thickness of the transparent layer. An example of the first record carrier is the so-called DVD, having a transparent layer of 0.6 mm thickness and designed for being scanned at a wavelength between 630 and 650 nm. An example of the second record carrier is the so-called CD, having a transparent layer of 1.2 mm thickness and designed for being scanned at a wavelength between 780 and 820 nm.
The transparent layer, through which a radiation beam scans the information layer, introduces a so-called spherical aberration in the radiation beam. The spherical aberration is compensated in the objective lens, making the radiation beam near its focus substantially free from spherical aberration. If an objective lens compensated for a first thickness of the transparent layer is used for scanning a record carrier with a transparent layer of a second thickness, the focus will be deteriorated due to the under- or over-compensated spherical aberration.
A device for scanning optical record carriers having transparent layers of different thicknesses is disclosed from the U.S. Pat. No. 5,615,200. This known device uses a radiation source comprising two diode lasers at different distances from the objective lens. A high-density record carrier having a thin substrate is scanned using a first vergence of the radiation beam, while a low-density record carrier having a thick substrate is scanned using a second vergency of the radiation beam. The change in vergency is achieved by using two diode lasers emitting at the same wavelength but positioned at different distances from the objective lens. The objective lens is designed for optimum performance when scanning the high-density record carrier. A disadvantage of the known device is that its performance is unsatisfactory when scanning the low-density record carrier.